1. Field of Invention
The present general inventive concept relates to the preparation and use of silicon-based materials with thermoelectric properties.
2. Description of the Related Art
The biggest sources of waste heat include electrical generators, steel and glass production, the use of combustible fuels for transportation, oil refineries, as well as the loss and cost of disposal of heat in terms of fans and heat sinks for electronic semiconductors and power devices, air conditioning, and other low temperature applications. The electrical efficiency of power plants for instance, the ratio between energy used in the process and the energy extracted is about 30%.
Physics offers a path for recovery of most all waste heat, from low temperature to high temperature, by exploiting the use of semiconductive materials exhibiting the Seebeck effect where electrons are both carriers of electricity and heat. This class of material is called thermoelectrics. Both n-type and p-type thermoelectrics must be used in tandem to produce power from waste heat or to (reversibly) use power to instead cool or heat through solid-state means (i.e., no moving parts except electrons). When a uniaxial temperature gradient is present within a TE structure, a colder and a warmer, a diffusion of electrons is created from the hot side toward the cold side, forming two poles of an electrical field. The voltage measured therein called Seebeck voltage. The Seebeck coefficient (S) has units of voltage per unit temperature and is a material property.
There exists a need to find a material for use as a thermoelectric semiconductor capable of being molded or fabricated into shapes to exploit a unidirectional thermal gradient for the production of electrical power.
Most contemporary thermoelectric materials and their compounds suffer from a variety of ills, including high cost, difficulty in processing, poor performance, use of precious or potentially carcinogenic elements, limited freedom of the engineer to manipulate properties such as electrical resistivity and thermal conductivity, thermal shock susceptibility, shape, size, indexing these properties from the hot side to the cold side and many others. For this reason, progress has been slow and unaccretive with regard to ZT and operating temperature and size for a very long time. None of those in the current road map use my approach of nano silicon porous structures, though all advances as cited above use nanostructuring to realize the benefits of the quantum size effects on thermal conductivity.
Materials such as bismuth telluride (Bi2Te3) and bismuth selenide (Bi2Se3) comprise some of the best performing low-temperature temperature thermoelectrics with a temperature-independent ZT between 0.8 and 1.0. These materials are used for solid-state cooling in small consumer refrigerators and for solid-state heating in some styles of mosquito traps. Nanostructuring these materials to produce a layered superlattice structure of alternating Bi2Te3 and Bi2Se3 layers produces a device within which there is good electrical conductivity but perpendicular to which thermal conductivity is poor. The result is an enhanced ZT (approximately 2.4 at room temperature for p-type), but this high value has not been independently substantiated. However, the availability of elements such as tellurium and selenium may soon be in jeopardy as reports indicate both are past their production peaks. Therefore it is desirable to identify thermoelectric materials having long-term abundance (e.g., silicon).
Skutterudite thermoelectrics are of contemporary interest because of their medium to high-temperature use capability. These structures are of the form (Co,Ni,Fe)(P,Sb,As)3. Unfilled, these materials contain voids into which low-coordination ions (usually rare earth elements) can be inserted to alter thermal conductivity by producing sources for lattice phonon scattering and decrease thermal conductivity due to the lattice without reducing electrical conductivity. However, the processing of making dense skutterudite thermoelectrics is not trivial. It involves powder preparation methods and billet formation methods that are not particularly well-suited for mass production or net shape processing. Al2O3 while being inexpensive and a low dielectric, has a high CTE and a low thermal conductivity.
BeO has a high thermal conductivity, low dielectric, but is toxic and expensive and has a high CTE.
AlN has a low dielectric, and high thermal conductivity and a low CTE, but is expensive.
Glass, including fused silica, is inexpensive, has a low dielectric, a low CTE and a very low thermal conductivity.
To date, silicon has not been widely used, possibly because of the danger of milling silicon to sizes in the micron to submicron range. While many of the factors for silicon have been proven, for instance ability to engineer low thermal conductivity, dope to increase electrical conductivity, relative insensitivity of silicon's Seebeck Coefficient to doping and electrical conductivity, high operating temperature, the approaches taken so far are mostly expensive, size limiting and process limiting approaches such as nano wires and MEMS.